1. Field of the Invention
The present invention relates to protecting power semiconductor element from overvoltage.
2. Description of the Background Art
To effectively utilize electrical power energy, a power conversion device that employs a power semiconductor element is generally used and for example, a power electronics device including a power conversion device for driving a motor includes an inverter device and the like.
This inverter device has a power semiconductor element therein, and in recent years, this semiconductor element is generally implemented by an insulated gate bipolar transistors (IGBT).
This power semiconductor element, or IGBT, is used in a method as a switching element to switch a current rapidly to control higher voltage and larger current. For such devices as the inverter device, when the semiconductor element switches, a current has a significant variation, which will hereinafter be represented in degree as a current gradient dI/dt, and a large surge voltage attributed to a floating inductance of a circuit will be applied to the semiconductor element.
If this surge voltage causes a current/voltage locus in switching to exceed the safe operation area (SOA) of the semiconductor element, the element is destroyed.
Accordingly, a variety of systems that alleviate or prevent such destruction of an element that is caused by surge voltage have conventionally been proposed, for example, as described in Japanese Patent Laying-open Nos. 04-354156, 2000-324797, 01-280355, 07-288456, 2002-135973, 2005-295653and 2006-042410.
As has been described above, a surge voltage increases depending on current gradient dI/dt provided when the semiconductor element switches. Accordingly, if a current is interrupted slowly, the surge voltage can be reduced. This, however, is tradeoff with increased switching loss.